This invention relates to gas turbines and, more particularly, to a method and apparatus for assembling a variable area turbine.
It is by now well understood that in gas turbine engines energy is added to the air through the processes of compression and combustion, while energy is extracted by means of a turbine. In a turbofan engine, compression is accomplished sequentially through a fan and thereafter through a multistage compressor, the fan and compressor being independently driven by a high pressure and a low pressure turbine, respectively, through concentric shaft connections. Combustion occurs between the multistage compressor and the high pressure turbine. Since the energy available to the turbines far exceeds that required to maintain the compression process, the excess energy is exhausted as high velocity gases through one or more nozzles at the rear of the engine to produce thrust by the reaction principle.
Since the fan and compressor are on separate concentric shafts and are driven by separate, axially spaced turbines, a means for regulating their relative rotational speeds is desirable for performance optimization. Further, it becomes desirable to control the relative amounts of energy added by the fan and compressor which, in turn, are controlled by how much energy is extracted by their respective turbines. It can be appreciated that the faster the fan or compressor rotates, the more air it pumps, and vice versa. Furthermore, it is recognized that if a stage of stationary turbine vanes may be made to provide a variable flow area through the turbine by making the vanes rotatable about their respective longitudinal axes, the energy extraction characteristics of either the high or low pressure turbines may be modulated. Thus, if the capability of the high pressure turbine to extract energy was reduced, more energy would be available to the low pressure turbine and the fan could be driven at a higher rotational speed relative to the compressor, and vice versa. This ability to regulate the relationship between fan and compressor rotational speeds is extremely important in designing the most efficient engine over a range of operating conditions. Such optimized engines have recently been referred to as variable cycle engines and are characterized as possessing variable geometry components in order to optimize performance for both subsonic and supersonic cruise, for example. It is characteristic of some of these variable cycle engines that both the high and low pressure turbines are of the variable area variety for maximum modulation of energy extraction.
Additionally, it has been the experience that when it becomes necessary to design a close-coupled, fully variable, single-stage, low-pressure turbine, two structural characteristics usually results. First, the variable area vanes are cantilevered from a structural frame using cylindrical trunnions on the outer end of the vanes, and the trunnions are installed by sliding them radially outwardly through holes in the frame which journal the trunnion for vane rotation. The second characteristic is a result of the first. That is, the vane inner support structure must be put on in two halves, one half of which is inserted from ahead of the vane row and the other half which is inserted from the rear, capturing the vane ends therebetween. On one particular engine, this was done with two matched inner structure halves each containing 42 matched hemispherical impressions to entrap 42 uniballs, one on the end of each of 42 vanes, therebetween. This proved to be a very expensive configuration which was not amenable to mass production.
Therefore, a turbine structure is required which can support the inner ends of variable area turbine vanes which are cantilevered from an outer structural frame and which does not require the use of expensive uniballs and matched assemblies.